Repulsive Van Der Waals Forces Research Articles

Tailored twisted CNT bundle with improved inter-tube slipping performances

The exceptional mechanical properties of carbon nanotubes (CNTs) have encouraged the development of high-performance synthetic fibers in composite materials. To understand the effect of twisting on the mechanical and slipping performances of CNT bundles, molecular dynamics simulation is applied to investigate the tensile performances, failure modes, and pull-out responses of twisted CNT bundles. A molecular model comprising nineteen parallel aligned single-walled carbon nanotubes (SWCNTs) is twisted into bundles at angles of 0°, 10°, and 20°, and further tensile and pull-out simulations are performed. The tensile simulation indicates that compared with the non-twisted CNT bundle showing a tensile strength of about 82 GPa with obvious inter-tube slipping, the 10°-twisted bundle exhibits a tensile strength of approximately 70 GPa with SWCNTs fracture as the main failure mode, which indicates that twisting improves the inter-tube slipping performance without causing excessive strength reduction. Comparatively, when the twisting angle is 20°, no inter-tube slipping is observed and the tensile strength of the CNT bundle is measured to be 55 GPa, which decreases by approximately 32.9 %. The pull-out simulations further reveal that the pull-out forces increase as the twisting angle increases and the weakened bundle strength of twisted bundle is attributed to the repulsive van der Waals forces caused by the reduced distances between inter-tubes. Essentially, twisting is unfavorable for the overall mechanical strength while torsional densification mitigates the inter-tube slipping, which indicates that a trade-off need to be achieved. This paper provides insights into the tensile and slipping performances of twisted CNT bundles and forms a basis for enhancing the assembled CNTs bundle as the next-generation reinforcing phase in composite materials.

How to More Effectively Obtain Ginsenoside Rg5: Understanding Pathways of Conversion.

Ginsenoside Rg5, a relatively uncommon secondary ginsenoside, exhibits notable pharmacological activity and is commonly hypothesized to originate from the dehydration of Rg3. In this work, we compared different conversion pathways using Rb1, R-Rg3 and S-Rg3 as the raw material under simple acid catalysis. Interestingly, the results indicate that the conversion follows this reaction activity order Rb1 > S-Rg3 > R-Rg3, which is contrary to the common understanding of Rg5 obtained from Rg3 by dehydration. Our experimental results have been fully confirmed by theoretical calculations and a NOESY analysis. The DFT analysis reveals that the free energies of S-Rg3 and R-Rg3 in generating carbocation are 7.56 mol/L and 7.57 mol/L, respectively, which are significantly higher than the free energy of 1.81 mol/L when Rb1 generates the same carbocation. This finding aligns with experimental evidence suggesting that Rb1 is more prone to generating Rg5 than Rg3. The findings from the nuclear magnetic resonance (NMR) analysis suggest that the fatty chains (C22-C27) in R-Rg3 and S-Rg3 adopt a Gauche conformation and an anti conformation with C16-C17 and C13-C17, respectively, due to the relatively weak repulsive van der Waals force. Therefore, the configuration of R-Rg3 is more conducive to the formation of intramolecular hydrogen bonds between 20C-OH and 12C-OH, whereas S-Rg3 lacks this capability. Consequently, this also explains the fact that S-Rg3 is more prone to dehydration to generate Rg5 than R-Rg3. Additionally, our research reveals that the synthetic route of Rg5 derived from protopanaxadiol (PPD)-type ginsenosides (including Rb1, Rb2, Rb3, Rc and Rd) exhibits notable advantages in terms of efficacy, purity and yield when compared to the pathway originating from Rg3. Moreover, this study presents a highly effective and practical approach for the extensive synthesis of Rg5, thereby facilitating the exploration of its pharmacological properties and potential application in drug discovery.

Spectroscopic, DFT study, and molecular docking investigation of N-(3-methylcyclohexyl)-2-phenylcyclopropane-1-carbohydrazide as a potential antimicrobial drug

Detailed theoretical molecular modeling, spectral (NMR, FT-IR, and UV–vis) analysis, electronic properties, and in silico biological study of N-(3-Methylcyclohexyl)-2-phenyl cyclopropane-1-carbohydrazide (PI2) has been studied in this present research using experimental and electronic structure theory calculations based on density functional theory (DFT). While the quantum calculations were done using DFT method at the B3LYP/6-31+G(d) level of theory, the characterization of PI2 was carried out with 1HNMR, UV-vs, and FT-IR spectroscopy. The major vibrations recorded for the studied compound were those of amide, CC, C–C, N–H, and C–H stretch. The UV–visible excited states calculation using Time Dependent Density Functional Theory (TD-DFT) showed the highest orbital contributions in S0 – S5 with π→π* assignment for all the studied phases. The visual study of weak interaction (NCI), nonlinear optics, and the natural bond orbital (NBO) results showed majorly a steric effect due to strong repulsive forces and van der Waals forces, a better NLO property in the studied compound as compared with urea (standard) and the intramolecular charge transfer in the studied compound showed the highest energy of stabilization of 66.79KCal/Mol for the donor – acceptor interaction of LP (1) N23 → π*C25 - O26 respectively. Molecular docking investigation of PI2 was achieved using six (6) target proteins from which two were gram negative; Escherichia coli (2NYU) and Pseudomonas aeruginosa (7CID), 2 g positive; Staphylococcus aureus (6P9J) and Streptococcus pyogenes (1MG1), one helminth; Ascaris lumbricoides (3FJU) and one fungus; Candida albican (6TZ6). The PI2 docked results were compared with the docked results obtained for the standard drugs (Ciprofloxacin, Albendazole, and Fluconazole). The studied structure showed the highest binding affinity of −8.5 kcal/Mol for 1MG1 as compared with −6.9 kcal/Mol for that of the standard drug. Finally, ADMET properties of PI2 were computed and the result showed a better drug property of the studied compound.

Bouncing dynamics of electrostatically actuated NEM switches

The aim of the present research is to understand the bouncing dynamic behavior of nanoelectromechanical (NEM) switches in order to improve switch performance and reliability. It is well known that bouncing can dramatically degrade the switch performance and life; hence, in the present study, the bouncing dynamics of a cantilever-based NEM switch has been studied in detail. To this end, the repulsive van der Waals force is incorporated into a nano-switch model to capture the contact dynamics. Intermolecular forces, surface effects, and gas rarefication effects were also included in the proposed model. The Euler–Bernoulli beam theory and an approximate approach based on Galerkin’s method have been employed to predict transient dynamic responses. In the present study, performance parameters such as initial contact time, permanent contact time, major bounce height, and the number of bounces, were quantified in the presence of interactive system nonlinearities. The performance parameters were used to investigate the influence of surface effects and rarefication effects on the performance of an electrostatically actuated switch. Recommended operating conditions are suggested to avoid excessive bouncing for these types of NEM switches.

Optical van-der-Waals forces in molecules: from electronic Bethe-Salpeter calculations to the many-body dispersion model

Molecular forces induced by optical excitations are connected to a wide range of phenomena, from chemical bond dissociation to intricate biological processes that underpin vision. Commonly, the description of optical excitations requires the solution of computationally demanding electronic Bethe-Salpeter equation (BSE). However, when studying non-covalent interactions in large-scale systems, more efficient methods are desirable. Here we introduce an effective approach based on coupled quantum Drude oscillators (cQDO) as represented by the many-body dispersion model. We find that the cQDO Hamiltonian yields semi-quantitative agreement with BSE calculations and that both attractive and repulsive optical van der Waals (vdW) forces can be induced by light. These optical-vdW interactions dominate over vdW dispersion in the long-distance regime, showing a complexity that grows with system size. Evidence of highly non-local forces in the human formaldehyde dehydrogenase 1MC5 protein suggests the ability to selectively activate collective molecular vibrations by photoabsorption, in agreement with recent experiments.

Transport and retention of copper oxide nanoparticles under unfavorable deposition conditions caused by repulsive van der Waals force in saturated porous media

Currently, copper oxide nanoparticles (CuO NPs) have been widely used in industry, manufacturing and agriculture. The transport and retention of CuO NPs are vital to understanding the fate as well as the life cycle of CuO NPs in the environment. This study systematically investigates the transport and retention of CuO NPs in saturated porous media, and the experimental results were explained by the CFT and DLVO theory. The van der Waals force between CuO NPs and collector was repulsive, resulting in the unfavorable deposition condition. Column experiments were conducted with saturated quartz sand under environmentally relevant pH (6, 8, 9), ionic strength (IS, 1, 10, 50 mM), and humic acid (HA, 0.1–10 mg-C/mL). Experimental results show that the breakthrough curves (BCTs) were affected by different pH and IS. Under pH 6 and 9, the mobility of CuO NPs was enhanced by high IS while the mobility was inhibited by high IS under pH 8. The mobility of CuO NPs was enhanced by humic acid and the effect was best at 0.5 mg-C/mL HA. The experimental results were successfully explained by CFT and DLVO theory, the main mechanisms were aggregation of CuO NPs, interaction energy and collision between CuO NPs and collector. In general, these findings can improve our understanding of the transport and retention of CuO NPs in subsurface environments, and suggest pH, IS, HA may be key factors governing mobility and stability of CuO NPs in natural environment.

Topochemical engineering of composite hybrid fibers using layered double hydroxides and abietic acid.

Topochemical engineering of hybrid materials is an efficient way of synthesizing hydrophobic and highly tensile fiber composites by utilizing the intermolecular hydrogen bonds in natural materials. These materials include wood pulp fibers, abietic acid (resin acid) and inexpensive metal salts. In this work, a hybrid composite was created using bleached and unbleached kraft pulp fibers as cellulose platform. In situ co-precipitation of layered double hydroxide (LDH) was performed to grow LDH crystals on the surface of the cellulose fibers, followed by the immobilization of abietic acid (AA) on LDH-grafted cellulose. Here we aimed to benefit from the hydrogen bonding between –OH groups of cellulose and LDH, and the –COOH groups of AA to obtain charge-directed assembly of one material on the other material. Thus, composite hybrid fibers (C-HF) were produced and then characterized by optical (CAM), spectroscopic (XRD, IR) and microscopic techniques (SEM) to determine their average length and distribution, structure and purity, bonding, and morphology. These fibers further were tested for water contact angle (hydrophobicity), oil absorption (lipophilicity), tensile strength and ISO brightness measurements. The performance of C-HF was compared with unmodified reference fibers (REF), fibers composed with only AA (C-F) and LDH-hybridized fibers (HF). The results revealed a variety of correlations between materials and their properties due to characteristic surface morphology, functional groups, hydrogen bonding and natural co-materials such as lignin and hemicelluloses. Attractive and repulsive van der Waals forces between material entities play a crucial role in the resulting properties.

First-Principles Investigation on the Tribological Properties of h-BN Bilayer Under Variable Load

Using first-principles method, for h-BN bilayer, we successfully probe the major factors of different low-friction paths in the three-dimensional potential energy surface (3D-PES) under variable loads. By means of the static PES and charge density difference analysis, we demonstrate how electrostatic interactions, with regard for van der Waals contributions at 0 nN, progressively impact the shape of 3D-PES and low-friction paths with increasing the normal load. Herein, the sliding properties of h-BN bilayers have a distinct relative orientation. Especially, the load-induced 3D-PES with variable shape is assigned to the band gap and repulsive van der Waals force. It is noted that the low friction not only is obtained for the commensurate layers under low loads, but also high ones.

Ice Particles Sink below the Water Surface Due to a Balance of Salt, van der Waals, and Buoyancy Forces

According to the classical Archimedes’ principle, ice floats in water and has a fraction of its volume above the water surface. However, for very small ice particles, other competing forces such as van der Waals forces due to fluctuating charge distributions and ionic forces due to salt ions and charge on the ice surface also contribute to the force balance. The latter crucially depends on both the pH of the water and the salt concentration. We show that a bulge in the air–water interface due to interaction of surface tension with the rising ice particle becomes significant when the particle radius is greater than 50–100 μm. The role of these forces in governing the initial stages of ice condensation has never been considered. Here, we show that small ice particles can only form below an exclusion zone, from 2 nm (in high salt concentrations) up to 1 μm (in pure water at pH 7) thick, under the water surface. This distance is defined by an equilibrium of upward buoyancy forces and repulsive van der Waals forces. Ionic forces due to salt and ice surface charge push this zone further down. Only after growing to a radius larger than 10 μm, will the ice particles eventually float toward the water surface in agreement with the simple intuition based on Archimedes’ principle. Our result is the first prediction of observable repulsive van der Waals forces between ice particles and the water surface outside a laboratory setting. We posit that it has consequences on the biology of ice water as we predict an exclusion zone free of ice particles near the water surface which is sufficient to support the presence of bacteria.

Hydration film measurement on mica and coal surfaces using atomic force microscopy and interfacial interactions

The hydration film on particle surface plays an important role in bubble-particle adhesion in mineral flotation process. The thicknesses of the hydration films on natural hydrophobic coal and hydrophilic mica surfaces were measured directly by atomic force microscopy (AFM) based on the bending mode of the nominal constant compliance regime in AFM force curve in the present study. Surface and solid-liquid interfacial energies were calculated to explain the forming mechanism of the hydration film and atomic force microscopy data. The results show that there are significant differences in the structure and thickness of hydration films on coal and mica surfaces. Hydration film formed on mica surface with the thickness of 22.5 nm. In contrast, the bend was not detected in the nominal constant compliance regime. The van der Waals and polar interactions between both mica and coal and water molecules are characterized by an attractive effect, while the polar attractive free energy between water and mica (−87.36 mN/m) is significantly larger than that between water and coal (−32.89 mN/m), which leads to a thicker and firmer hydration layer on the mica surface. The interfacial interaction free energy of the coal/water/bubble is greater than that of mica. The polar attractive force is large enough to overcome the repulsive van der Waals force and the low energy barrier of film rupture, achieving coal particle bubble adhesion with a total interfacial free energy of −56.30 mN/m.

Role of Collectors and Depressants in Mineral Flotation: A Theoretical Analysis Based on Extended DLVO Theory

A theoretical analysis was conducted to study the role of collectors and depressants in flotation, based on the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, where the hydrophobic force is considered. The collector-coated hydrophilic particle and the depressant-coated hydrophobic particle are simplified to a sphere uniformly covered with respectively hydrophobic and hydrophilic nanometer-sized hemispherical asperities of identical radius. Results show that the role of a collector in bubble–particle attachment is to create an attractive hydrophobic force and thus overcome the repulsive van der Waals and electrostatic forces. Moreover, increasing the length of the hydrophobic part of the collector molecule is a more effective way to enhance flotation recovery, compared to increasing the collector concentration. For a depressant, however, its function mechanism is to create a strong electrostatic double-layer force, while the suppression of the hydrophobic force plays a secondary role in decreasing the bubble–particle attachment barrier. The depressant molecule length is also a dominant parameter in designing a powerful depressant.

Measurement of hydrophobic forces in thin liquid films of water between bubbles and xanthate-treated gold surfaces

In flotation, bubble-particle attachment occurs when the wetting films of water formed in between are destabilized by collector coating. In most cases, DLVO forces are repulsive and hence stabilize wetting films; therefore, it is necessary to consider the role of attractive hydrophobic force as a destabilizer. In this regard, hydrophobic forces have been measured by monitoring dynamic profiles of an air bubble in nanoscale as it approaches a flat surface using the method described previously (Pan et al., 2011). The measurements conducted with gold substrates contacted in a 10−5M potassium ethyl xanthate (KEX) show that hydrophobic force increases with increasing contact time and water contact angle. After an excessively long contact time, hydrophobic force is found to decrease possibly due to multi-layer adsorption of xanthate.The forces measured from dynamic film profiles are in good agreement with those measured directly using a specially designed force apparatus. Analysis of the force data using the Frumkin-Derjaguin isotherm shows that bulk of the energy gained during bubble-particle interaction is due to the work done by the hydrophobic force at short range to overcome the repulsive van der Waals force. At long-range, hydrophobic force facilitates the kinetics of film thinning by reducing the force barrier created by double-layer force.

Self-Limitation of Native Oxides Explained

Silicon is one of many materials whose surface will oxidize in ambient conditions. However it is one of few materials whose native oxide will self-limit its growth in a matter of hours at a thickness of ∼2 nm. In this work, we show through the theory of repulsive van der Waals forces that this self-limitation is due, at least in part, to the interaction between the inherent material properties of a native silicon oxide film on silicon and oxidizing molecules. These molecules are not just hindered from even entering the system at all, but those that do enter the native oxide film are repelled away from the silicon – silicon oxide interface, preventing additional growth by oxidation. We also show how this repulsion is overcome by increasing ambient temperatures to subsequently increase the kinetic energy of the oxidizing molecules, calculated by the Boltzmann-Maxwell distribution, and allow oxidation to continue.

Microwaves Probe Dipole Blockade and van der Waals Forces in a Cold Rydberg Gas.

We show that microwave spectroscopy of a dense Rydberg gas trapped on a superconducting atom chip in the dipole blockade regime reveals directly the dipole-dipole many-body interaction energy spectrum. We use this method to investigate the expansion of the Rydberg cloud under the effect of repulsive van der Waals forces and the breakdown of the frozen gas approximation. This study opens a promising route for quantum simulation of many-body systems and quantum information transport in chains of strongly interacting Rydberg atoms.

Repulsive van der waals forces self-limit native oxide growth.

Silicon is one of the most studied materials, yet questions remain unanswered about its unusual property of growing a self-limiting native oxide that attains its final thickness in a matter of hours yet months later has not grown further. For the first time, we have explored this self-limiting growth in terms of repulsive van der Waals (vdW) forces generated by the combination of material properties inherent to the system. These repulsive forces represent an energy barrier preventing additional oxidizing chemicals, mainly oxygen and water, from adsorbing on the surface as well as hindering diffusion of those that do adsorb toward the interface. We have also proven that this native oxide can be increased in thickness at room temperature and without reactive species by changing the oxidation environment to one predicted by theory to result in attractive vdW forces, thus allowing oxygen/water to interact with the surface more freely.

Solid state 13C-NMR, infrared, X-ray powder diffraction and differential thermal studies of the homologous series of some mono-valent metal (Li, Na, K, Ag) n-alkanoates: A comparative study

A comparative study of the molecular packing, lattice structures and phase behaviors of the homologous series of some mono-valent metal carboxylates (Li, Na, K and Ag) is carried out via solid state FT-infrared and 13C-NMR spectroscopes, X-rays powder diffraction, density measurements, differential scanning calorimetry, polarizing light microscopy and variable temperature infrared spectroscopy. It is proposed that, for lithium, sodium and potassium carboxylates, metal-carboxyl coordination is via asymmetric chelating bidentate bonding with extensive intermolecular interactions to form tetrahedral metal centers, irrespective of chain length. However, for silver n-alkanoates, carboxyl moieties are bound to silver ions via syn–syn type bridging bidentate coordination to form dimeric units held together by extensive head group inter-molecular interactions. Furthermore, the fully extended hydrocarbon chains which are crystallized in the all-trans conformation are tilted at ca. 30°, 27°, 15° and 31° with respect to a normal to the metal plane, for lithium, sodium, silver and potassium carboxylates, respectively. All compounds are packed as lamellar bilayer structures, however, lithium compounds are crystallized in a triclinic crystal system whilst silver, sodium and potassium n-alkanoates are all monoclinic with possible P1 bravais lattice. Odd–even alternation observed in various physical features is associated with different inter-planar spacing between closely packed layers in the bilayer which are not in the same plane; a phenomenon controlled by lattice packing symmetry requirements.All compounds, except silver carboxylates, show partially reversibly first order pre-melting transitions; the number of which increases with increasing chain length. These transitions are associated, for the most part, with lamellar collapse followed by increased gauche-trans isomerism in the methylene group assembly, irrespective of chain length. It is proposed that the absence of mesomorphic transitions in their phase sequences is due to a lack of sufficient balance between attractive and repulsive electrostatic and van der Waals forces during phase change. The evidence presented in this study shows that phase behaviors of mono-valent metal carboxylates are controlled, mainly, by head group bonding.

Predicting and measuring repulsive van der Waals forces for a Teflon AF™–solvent–α-alumina system

The force interaction of an α-alumina particle against Teflon AF™ in the presence of several non-polar and polar solvents was quantified. Solvents were chosen based on calculation of a negative Hamaker constant, indicating a repulsive van der Waals interaction. These results were compared to those predicted by theoretical models for van der Waals forces. Measured forces for the non-polar solvent interactions are comparable to theoretical predictions, whereas the polar solvent interactions show repulsive forces between one and two orders of magnitude smaller than those predicted. In the case of water, surface charges were large enough to completely counteract the repulsive van der Waals forces and lead to an attractive force interaction. This minimization of repulsion, and even attraction, can be predicted by DLVO theory.

Stabilization of thin liquid films by repulsive van der Waals force.

Using high-speed video recording of bubble rise experiments, we study the stability of thin liquid films trapped between a rising bubble and a surfactant-free liquid-liquid meniscus interface. Using different combinations of nonpolar oils and water that are all immiscible, we investigate the extent to which film stability can be predicted by attractive and repulsive van der Waals (vdW) interactions that are indicated by the relative magnitude of the refractive indices of the liquid combinations, for example, water (refractive index, n = 1.33), perfluorohexane (n = 1.23), and tetradecane (n = 1.43). We show that, when the film-forming phase was oil (perfluorohexane or tetradecane), the stability of the film could always be predicted from the sign of the vdW interaction, with a repulsive vdW force resulting in a stable film and an attractive vdW force resulting in film rupture. However, if aqueous electrolyte is the film-forming bulk phase between the rising air bubble and the upper oil phase, the film always ruptured, even when a repulsive vdW interaction was predicted. We interpret these results as supporting the hypothesis that a short-ranged hydrophobic attraction determines the stability of the thin water film formed between an air phase and a nonpolar oil phase.

A Model for the Stability of a TiO2 Dispersion

A computational study of a colloidal dispersion stabilized with grafted polymer layers is presented here as a model for white, water-based paints. The interaction model includes repulsive, three-body interactions and attractive van der Waals forces. The electrostatic interactions are also studied. Stability criteria can be established for the dispersion, such as the thickness of the adsorbed polymer layers, and the quality of the solvent. Using implicit solvent molecular dynamics calculations, the spatial distribution of the pigments is obtained through the calculation of the radial distribution functions. The results show that the solvent quality and the thickness of the grafted polymer layer are key variables in the stability of the dispersion. Additionally, a structural phase transition is predicted, which is driven by the pigment concentration in the dispersion. It is argued that the predictions of this work are useful guidelines in the design of paints and coatings of current industrial interest.

Repulsive van der Waals forces due to hydrogen exposure on bilayer graphene

We consider the effect of atomic hydrogen exposure to a system of two undoped sheets of graphene grown near a silica surface (the first adsorbed to the surface and the second freestanding near the surface). In the absence of atomic hydrogen, the van der Waals force between the sheets is attractive at all separations, causing the sheets to come closer together. However, with the addition of atomic hydrogen between the sheets, the long-range van der Waals interaction turns repulsive at a critical concentration. The underlying triple layer structure (SiO${}_{2}$--atomic hydrogen gas--air) gives rise to a long-range repulsion that at large-enough separations dominates over the more rapidly decaying attraction between the two-dimensional undoped graphene sheets (and between the outer graphene sheet and SiO${}_{2}$). This may be an avenue to tune the separation between two graphene sheets with the gas concentration. The doping of the graphene layers increases the attractive part of the interaction and hence reduces the net repulsive interaction.